The present invention relates to a copier or similar image forming apparatus and, more particularly, to an image fixing device having a fixing roller implemented as a heat pipe and a pressing roller held in pressing contact with the fixing roller.
It is a common practice with a copier or similar electrophotographic image forming apparatus to electrostatically transfer a toner from a photoconductive element to a paper, fix the toner image on the paper by an image fixing unit, and then discharge the paper, or copy, to the outside of the copier. In the image fixing device, a fixing roller is heated while a pressing roller is held in pressing contact with the fixing roller. The two rollers fix the toner image on the paper by heat and pressure while conveying the paper in cooperation. It has been customary to heat the fixing roller by a heater accommodated in the fixing roller and extending in the axial direction of the roller. In this type of fixing device, since the fixing roller is subjected to heat and pressure, the fixing ability thereof is sequentially lowered due to, for example, contamination by the toner. Moreover, the heater disposed in the fixing roller is apt to snap. In the event of such an occurrence, the fixing roller is replaced with a new fixing roller. However, the problem is that the replacement of the fixing roller is time- and labor-consuming due to conducting means arranged to feed power to the heater.
In the light of the above, Japanese Patent Laid-Open Publication No. 4-32885 teaches an image fixing device having a fixing roller constituted by a heat pipe which is open at one end thereof. A conical heat receiving tube is formed with a number of projections and recesses on the inner periphery thereof. The heat receiving tube is inserted in the open end of the heat pipe with the apex thereof facing the inside of the heat pipe. A heater unit is accommodated in the heat receiving unit while the heat pipe is filled with a working fluid. In this configuration, heat generated by the heater unit is transferred to the working fluid efficiently via the heat receiving tube, thereby heating the heat pipe or fixing roller. With this conventional device, it is possible to promote efficient heat transfer and efficient replacement, reduce a preheating time, and set up a uniform temperature distribution over the entire fixing roller.
However, the conventional fixing device described above as some problems left unsolved, as follows. To begin with, the fixing roller implemented by a heat pipe needs a heating portion having a substantial length. In addition, the heater unit uses a halogen heater as a heat source and, therefore, increases the diameter of the fixing roller. As a result, the fixing device and an image forming apparatus using it are bulky. Assume that the heating portion of the fixing roller has the length thereof reduced in order to miniaturize the overall image forming apparatus. Then, since a great amount of heat is locally conducted to the heating portion within a predetermined period of time, the heating portion is subjected to a temperature higher than 200.degree. C. In this condition, it is likely that the heating portion is heated to above the breakdown limit thereof and bursts.
It is a common practice to coat the outer periphery of a paper nipping portion, which forms part of the fixing roller, with a PFM or similar fluoric resin so as to promote easy parting. Usually, a fluoric resin layer is formed by applying a fluoric resin solution to the outer periphery of the paper nipping portion and then baking it at 350.degree. C. to 400.degree. C. for 20 minutes to 40 minutes. However, when the fixing roller filled with a working fluid, as stated above, is baked under such conditions, the pressure inside the cavity of the roller is apt to rise to a pressure high enough to cause the roller to burst.
Further, while the heat pipe is usually made of copper and provided with a wall thickness of 1.0 mm to 1.5 mm, it is apt to deform due to high temperature (150.degree. C. to 200.degree. C.) and pressure attributable to the pressing roller. As a result, the fixing ability available with the device is lowered at the center of the paper nipping portion of the fixing roller.
Moreover, heat has to be transferred from the heating portion to the paper nipping portion of the fixing roller within the cavity of the roller. To effect such heat conduction, the cavity of the roller, which is filled with the working fluid, is required to have a volume great enough for the fluid to evaporate and then condense. Hence, when heat generated by the heater unit is transferred to the fluid via the conical heat receiving tube inserted in the heat pipe, as stated earlier, the heating efficiency is low.